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Fire, not corn was key to prehistoric survival in the arid Southwest USA

Conventional wisdom holds that prehistoric villagers planted corn, and lots of it, to survive the dry and hostile conditions of the American Southwest.

But University of Cincinnati archaeology professor Alan Sullivan is challenging that long-standing idea, arguing instead that people routinely burned the understory of forests to grow wild crops 1,000 years ago.

"There has been this orthodoxy about the importance of corn," said Sullivan, director of graduate studies in UC's Department of Anthropology in the McMicken College of Arts and Sciences. "It's been widely considered that prehistoric peoples of Arizona between A.D. 900 to 1200 were dependent on it.

"But if corn is lurking out there in the Grand Canyon, it's hiding successfully because we've looked all over and haven't found it."

The Fruitless Search For Corn

Sullivan has published a dozen papers outlining the scarce evidence of corn agriculture at more than 2,000 sites where they have found pottery sherds and other artifacts of prehistoric human settlement. He summarized his findings in a presentation last month at Boston University.

Sullivan has spent more than two decades leading archaeological field research to Grand Canyon National Park and the region's Upper Basin, home to the 1.6-million-acre Kaibab National Forest.

When you think of the Grand Canyon, you might picture rocky cliffs and desert vistas. But the Upper Basin, where Sullivan and his students work, is home to mature forests of juniper and pinyon trees stretching as far as you can see, he said.

"When you look down into the Grand Canyon, you don't see any forest. But on either rim there are deep, dense forests," he said.

The rim of the Grand Canyon is filled with forests (Image: Alan Sullivan )

Forest Dwellers

On these high-elevation plateaus, Sullivan and his students have unearthed ceramic jugs adorned with corrugated patterns and other evidence of prehistoric life. Sullivan is particularly interested in the cultural and social practices of growing, sharing and eating food, also called a ‘foodway.’

"What would constitute evidence of a corn-based foodway?" he asked. "And if experts agree it should look like this but we don't find evidence of it, that would seem to be a problem for that model."

Like a detective, Sullivan has pieced together clues firsthand and from scientific analysis to make a persuasive argument that people used fire to promote the growth of edible leaves, seeds and nuts of plants such as amaranth and chenopodium, wild relatives of quinoa. These plants are called "ruderals," which are the first to grow in a forest disturbed by fire or clear-cutting.

"It's definitely a paradigm-threatening opinion," Sullivan said. "It's not based on wild speculation. It's evidence-based theorizing. It has taken us about 30 years to get to the point where we can confidently conclude this."

UC students work at many archaeological sites outside Grand Canyon National Park where they have found evidence of prehistoric civilization. (Image: Alan Sullivan )

Evidence from the Lab

Lab analysis identified ancient pollen from dirt inside clay pots that were used 1,000 years ago before Sullivan and his students found them.

"They've identified 6,000 or 7,000 pollen grains and only six [grains] were corn. Everything else is dominated by these ruderals," Sullivan said.

The corn itself looked nothing like the hearty ears of sweet corn people enjoy at barbecues today. The ears were puny, about one-third the size of a typical cob, with tiny, hard kernels, Sullivan said.

So if prehistoric people were not growing corn, what were they eating? Sullivan found clues around his excavation sites that people set fires big enough to burn away the understory of grasses and weeds but small enough not to harm the pinyon and juniper trees, important sources of calorie-rich nuts and berries.

Evidence for this theory was found in ancient trees. Raging wildfires leave burn scars in growth rings of surviving trees. In the absence of frequent small fires, forests would accumulate vast amounts of underbrush and fallen timber to create conditions ripe for an inferno sparked by a lightning strike. But examinations of ancient juniper and ponderosa pine trees found no burn scars, suggesting big fires are a relatively new phenomenon in Arizona.

Heavy equipment was used to remove dirt at an excavation site outside Grand Canyon National Park in Arizona. (Credit: Alan Sullivan )

Fanning the Flames of the Theory

Sullivan also studied the geologic layers at these sites. Like a time capsule, the stratigraphic analysis captured the periods before and after people lived there. He found higher concentrations of wild edible plants in the period when people lived there. And when people abandoned the sites, the area they left behind saw fewer of these plants.

But it was only this year that Sullivan found contemporary evidence supporting his theory that prehistoric people generated a spring bounty by setting fires. Sullivan returned to the Grand Canyon last spring to examine forest destroyed by a massive 2016 fire. Touched off by a lightning strike, the blaze called the Scott Fire laid waste to 2,660 acres of pines, junipers and sagebrush.

Support from the National Park Service

Archaeologists with the National Park Service have found evidence that corn grew below the rim of the Grand Canyon, said Ellen Brennan, cultural resource program manager for the national park.

"It does appear that the ancient people of the Grand Canyon never pursued corn agriculture to the extent that other ancestral Puebloan peoples did in other parts of the Southwest," Brennan said. "In the Grand Canyon, it appears that there continued to be persistent use of native plants as a primary food source rather than corn."

The National Park Service has not examined whether prehistoric people used fire to improve growing conditions for native plants. But given what is known about cultures at the time, it is likely they did, Brennan said.

The first assumptions about what daily life was like in the Southwest 1,000 years ago came from more recent observations of Native Americans such as the Hopi, said Neil Weintraub, archaeologist for Kaibab National Forest. He worked alongside Sullivan at some of the sites in the Upper Basin.

"Corn is still a big part of the Hopi culture. A lot of dances they do are about water and the fertility of corn," he said. "The Hopi are seen as the descending groups of Puebloan."

While native peoples elsewhere in the Southwest no doubt relied on corn, Weintraub said, Sullivan's work has convinced him that residents of the Upper Basin relied on wild food -- and used fire to cultivate it.

"It's a fascinating idea because we really see that these people were highly mobile. On the margins where it's very dry we think they were taking advantage of different parts of the landscape at different times of the year," Weintraub said.

"It's been well documented that Native Americans burned the forest in other parts of the country. I see no reason why they wouldn't have been doing the same thing 1,000 years ago," he said.

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